The "Pink Star", as it is known, is over twice the size of the 24.78 carat "Graff Pink" diamond that set the world auction record for $46.2m in 2010.
Mined by De Beers in Africa in 1999, it has the highest possible colour and clarity rating from the Gemological Institute of America (GIA).
Meanwhile, the author Barbara Taylor Bradford has announced that she will sell a collection of 40 items of jewellery through Bonhams in December. The 14 items are expected to make around £1.5m.
